We have observed a pyrrole pigment in bile, meconium and small intestines of newborn monkeys and in fetal humans prior to birth. This suggests that there is an enterohepatic circulation of tetrapyrroles in the perinatal period. The enterohepatic circulation of these compounds will be investigated. The origin of various bilirubins excreted in newborn bile and some of the enzymes involved in their metabolism, i.e. heme oxygenase and UDPG transferase, will also be studied. To study these pigments, we will use spectroscopic methods, thin layer and high pressure liquid chromatography. Radioactive isotopes will be used as precursors for tetrapyrrole synthesis in vivo and in vitro and for preparation of 3H labeled heme. Attempts will be made to elucidate the steps involved in neonatal bile pigment formation and possibly their absorption by the intestinal wall of neonates, particularly premature infants.